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3 Answers

Using mixture to manipulate fuel flow?

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General Aviation

Is it possible to manipulate the mixture in order to get a desired fuel flow...or is this practice bad for the engine?

For instance, I check my performance charts and determine that 9.2gph is consumed at 2350 RPMs...and 7.7gph is consumed at 2200 RPMs.

Now in order to maximize my travel time, I may want to travel at 2350 RPMs.  Couldn't I simply lean the mixture until the fuel flow gauge read 7.7gph at 2350 RPMs?  (Keep in my 7.7gph is the consumption rate the performance chart listed for 2200 RPMs.)

If I leaned the fuel out this much for 2350 RPMs, would the engine simply die?  In other words, is the listed fuel flow of 9.2gph required for the engine to run effiectively at 2350 RPMs? 

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3 Answers

  1. Wesley Beard on Jan 29, 2011

    The performance tables listed in most manufacturer’s Pilot’s Operating Handbook has the mixture set at “lean mixture” using the C-172N POH downloaded from this website.  The lean mixture is discussed on page 4-16 and states “the mixture should be leaned until engine RPM peaks and drops 25-50 RPM.  At lower powers it may be necessary to enrichen the mixture slightly to obtain smooth operation.”
    The point is this, the airplane is already at its most efficient mixture setting at 2350 with a fuel flow of 9.2GPH.  Any leaner and you could (1) severely damage the engine (2) cause the engine to quit.  The engine requires a certain ratio between air and fuel to operate smoothly. 
    You might find though, if you were flying at 2200 RPM and lean out for 7.7 GPH and mark the mixture somehow for comparison that when you bump up the power to 2350 with 9.2 GPH and reset the mixture that you are pretty close, if not exactly, where you were before.

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  2. Kent Shook on Jan 29, 2011

    The other thing to note is that the cruise performance tables are usually set to “best power mixture.” 
    What that means is, even if you are at 2350 RPM, if you lean to the point where you’re burning less than 9.2 gph, you are ALSO reducing the power the engine is producing, thus the airplane will slow down and you won’t get there as fast as you would have at 2350 RPM and 9.2 gph. TANSTAAFL! (There ain’t… free lunch.)
    If you’re trying to get somewhere, you have a choice of maximizing the time spent getting there, or the fuel flow – Or striking an appropriate balance. Since cruise speed varies with the cube root of power but fuel flow varies pretty much directly with power, in any airplane you should be able to reduce power from 75% to 65% and your cruise speed will only be reduced by 5% while your fuel burn will be reduced by 13% – A pretty good tradeoff, and one that may allow you to make your destination with one less fuel stop on a long trip, meaning you would get there faster, too!

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  3. James MacGregor CFI on Jan 31, 2011

    Also keep in mind that your ideal mixture settings are based on MANY factors, the temperature, hot air is less dense, the temperature of your cylinder heads, if they start heating up you want to run a little more rich, YOUR ALTITUDE, higher you go the more you can lean (and vise versa), best way is to lean to a EGT setting (check your POH).

    The fuel flow figures in your POH should be used for planning fuel burn on the ground for planning how much fuel you will burn on a flight. Once you get in the air you need to use more FEEL and PERCEPTIONS then calculations.

    Also a little trick, if you have a carbed engine and want to make sure you’re not running too lean, pull the carb heat and make sure you still get a good drop, if you don’t then it’s running too lean.

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